1. Field of the Invention
This invention relates to a low temperature ultrasonic microscope using such low temperature liquid as liquid nitrogen, liquid argon or liquid helium as an ultrasonic wave transmitting medium.
2. Related Art Statement
There is already practiced an ultrasonic microscope whereby a sample to be observed is two-dimensionally scanned with an ultrasonic wave beam and reflected waves or transmitted waves from the sample are received to form an ultrasonic wave image of the sample. In such ultrasonic microscope, in order to obtain more accurate picture image information from the sample, it is strongly required to elevate the resolution of the sample image to be higher.
The plane resolution of an ultrasonic microscope is determined by the wavelength of the sound wave in the ultrasonic wave transmitting medium. On the other hand, there is a relation of c=f. .lambda. among the sound velocity c, frequency f and wavelength .lambda. of the sound wave in the transmitting medium. Therefore, in order to elevate the resolution, that is, to make the wavelength of the sound wave small, it is considered to make the frequency f of the sound wave large or to use a transmitting medium of a small sound velocity c. In a conventional ultrasonic microscope, as water has been used as an ultrasonic wave transmitting medium, in order to elevate the resolution, there has been used a method of elevating the frequency f of the sound wave. However, as the absorption of the sound waves in the transmitting medium is proportional to the square of the frequency f of the sound wave, there is a limit to the elevation of the frequency in practice. That is to say, in order to obtain a picture image of sufficient S/N, it is necessary to receive reflected waves above a certain level from the sample. Therefore, with the elevation of the frequency, it is necessary to shorten the distance for which the sound waves propagate through the transmitting medium and to reduce the attenuation caused by the absorption. This means to make small the working distance of an acoustic lens, that is, to make small the radius of curvature of the acoustic lens.
In the ultrasonic microscope practiced at present, there are obtained a frequency of 1.5 to 2.0 GH.sub.z and a resolution of 0.7 to 0.5 .mu.m. The resolution on this level corresponds to the case of observing with an ordinary optical microscope of a lens radius of curvature of 50 to 30 .mu.m and a working distance of about 30 to 10 .mu.m. In order to realize a resolution above this level, it is necessary to make smaller the working distance of the lens. However, it is difficult in the precision in working the lens and in the use of the apparatus.
Thus, the resolution of the ultrasonic microscope using water as a transmitting medium has reached the same level as of the resolution of the optical microscope but, depending on the sample to be observed, in the case of observing such electronic device as, for example, a super LSI device or ceramic device, a resolution above that of the optical microscope is required and it is strongly required to develop an ultrasonic microscope of a higher resolution.
In order to attain this object, it is considered to use a transmitting medium lower in the sound velocity c or absorption than water. Such transmitting medium is such low temperature liquid as liquid nitrogen, liquid argon or liquid helium. An ultrasonic microscope apparatus using this low temperature liquid as a transmitting medium is mentioned on pages 1629 to 1637 of The Journal of The Acoustic Society of America, Vol. 67 (1980). In this known ultrasonic microscope apparatus, a sample stand is arranged in the bottom of an adiabatic container, a sample is fitted on this sample stand and, on the other hand, an acoustic lens is arranged above the sample to two-dimensionally scan the sample.
However, the above described known ultrasonic microscope apparatus has it as an object to confirm the resolution and therefore has defects that it is very difficult to position the sample and it is difficult to replace the sample.
In order to cope with it, the present applicant has suggested an ultrasonic microscope wherein a sample rod supporting a sample is removably inserted into an adiabatic container containing such low temperature liquid as liquid nitrogen through a sliding seal and gate valve.
Now, in case such low temperature liquid as liquid nitrogen is used as a transmitting medium, this low temperature liquid will be usually in a boiling state under the atmospheric pressure. There is a problem that, due to bubbles produced by this boiling, a vibration will be generated and a picture image will be fogged. In order to prevent the picture image from being fogged, while the lens is being scanned to take in the picture image, the boiling of such low temperature liquid as liquid nitrogen may be stopped. Therefore, in order to stop the boiling of the low temperature liquid, it is considered to seal the adiabatic container during the scanning so that the internal pressure may be elevated by the boiling of the low temperature liquid itself and the boiling may be stopped.
However, this method has such defect as in the following. That is to say, the sample rod is fixed by the sliding seal which fixes the sample rod with only the friction of the sample rod with an O-ring and is not high in the rigidity. Therefore, by the elevation of the internal pressure of the adiabatic container, the sample rod will be pushed up though slightly and the distance between the lens and sample will vary. That is to say, the focus will be out.
In order to avoid it, it is considered to adjust a leak valve so that the evaporation amount after the boiling stops may escape and the pressure may be constant during the scanning. However, in this method, it is difficult to stably reproduce the same state with the boiled amount of the low temperature liquid, the amount of the liquid and the other factors. Therefore, the focus adjustment of a submicron order required of the low temperature ultrasonic microscope can not be stably made.